The objective of the proposed studies are to obtain a better understanding of: (1) the mechanisms that regulate the relative high brain concentrations of two pteridines; the folates and biopterin, and (2) the functions of the folate in brain. Specifically, the relationship between the folate binding protein of choroid plexus and the biochemical mechanisms by which folates are transported form blood into cerebrospinal fluid thorugh the choroid plexus and into the brain will be investigated. To achieve these goals, the transport of radiolabeled pteridines into and out of the brain and cerebrospinal fluid in vivo, and into brain slices, in vitro, in both normal and folate deficient animals will be documented. Also, the folate binding protein of choroid plexus will be purified by chromatography. The brain enzyme dihydrofolate reductase, which reduces dihydropteridines, will be purified and characterized. Its affinity for 7,8 dihydrobiopterin, which is synthesized in brain and is the oxidized form of the cofactor involved in the rate limiting step in catecholamine and serotonin synthesis, will be tested. To be certain that dihydrofolate reductase and not dihydropteridine reductase reduces 7,8 dihydro biopterin, brain dihydropteridine reductase will be purified and tested for its ability to reduce 7,8 dihydrobiopterin. Finally, the mechanisms by which folates may contribute to the formulation of methionine and purines in brain will be investigated. The proposed studies should not only provide new information about pteridine homeostasis in brain but will privide information about pteridine function that has implications for fields ad diverse as neurotransmitter regulation and cancer chemotherapy (e.g., methotrexate brain toxicity).